This invention relates to the preparation of rigid cellular polyurethanes. This invention relates particularly to a method for reducing the friability of cellular polyurethanes derived from certain halogen-containing polyols.
Flame retardant, rigid polyurethane foams can be obtained by reacting a chlorine- or bromine-containing polyol with a polyfunctional isocyanate. Methods for preparing many types of halogen-containing polyols are known in the art. Adducts of 4,4,4-trichloro-1,2-epoxybutane with polyhydric alcohols, particularly carbohydrate-based materials have been recognized as particularly effective precursors for flame retardant rigid polyurethane foams. A method for preparing this latter class of halogenated polyols is described in U.S. Pat. No. 3,726,855, the disclosure of which is hereby incorporated by reference. When many of the aforementioned types of halogen-containing polyols are reacted with a polyfunctional isocyanate in the presence of conventional polymerization or gel catalysts, which include tertiary amines and compounds of divalent or tetravalent tin, such as stannous octoate and dibutyltin dilaurate, the resultant foams are often so friable that they cannot be handled without breaking or, in extreme instances, crumbling to a fine powder. The excellent flame retardancy exhibited by these foams is more than offset by this shortcoming and has delayed their acceptance in applications such as construction, which has a great need for these types of foams as insulating materials.
It is therefore an objective of this invention to increase the structural strength of rigid polyurethane foams derived from halogenated polyols. Surprisingly it has now been found that this objective is realized if the polymerization catalyst used to prepare the foam is a mixture of a dimethyltin-S,S'-bis(alkylmercaptocarboxylate) and a tertiary amine.